The invention herein described, relates to electrostatic precipitators, and more specifically, to improving the electrokinetic characteristics of the precipitator system and increasing its efficiency.
Electrostatic precipitators are used for dust collection in many fields, including: recovery of valuable products in dryers and smelters; collecting powdered products; in pneumatic conveying of spray dried milk, eggs and soap; cleaning air for areas used for the production of pharmaceutical products and photographic film; collecting pollutants for safety and health hazard elimination; and collecting fly-ash from power plant combustion gases.
When particles suspended in a gas are exposed to gas ions in an electrostatic field, they become charged and precipitate out under the action of the field. The functions involved in electrical precipitation include:
1. Gas ionization; and PA1 2. Particle collection, which is achieved by producing an electrostatic field to charge the dust particles, retaining the gas to permit particle migration to a collection surface, preventing re-entrainment of collected particles, and removing the collected particles.
The invention herein described concerns improving the gas ionization and electrostatic field production.
There are two general types of electrical precipitators, single-stage in which ionization and collection are combined, and two-stage in which the ionization is achieved in one zone and the collection in the other zone. The present invention is applicable to both of these types.
In order to obtain gas ionization it is necessary to exceed, at least locally, the electrical breakdown characteristic strength of the gas to produce corona. Sparking and arcing are advanced stages of corona in which complete breakdown of the gas occurs along a given discharge path. Both sparking and arcing undesirable and must be avoided.
Since corona represents a local breakdown, it can occur only in a non-uniform electrical field. For this reason, precipitators use irregular fields, generally with round or square wires suspended between flat plates. The fields are produced by applying to the wires and plates the highest voltage practicable without sparking or arcing. That provides maximum permissible particle charge and electrical precipitating field characteristics, thus increasing the overall efficiency of the precipitator. Corona discharge is accompanied by a relatively small flow of electric current, typically 0.1 to 0.5 milliamperes per square meter of collecting electrode area. Sparking and arcing usually involve a considerable larger flow of current which disrupts the operation, produces low collection efficiency because of the reduction in the applied voltage, causes redispersion of the collected particles, and damages the electrodes.
There are commercially available systems which regulate the current and voltage in precipitators and which tolerate a limited amount of arcing. Since the efficiency of the collection process is proportional to the average applied voltage, such systems attempt to maintain a substantially constant DC applied voltage that is just below the sparking threshold level. Of course, the voltage in such systems cannot continuously exceed the sparking threshold level. This limits the maximum efficiency attainable per unit area of collecting electrodes. Moreover, highly complex feedback apparatus is necessary to provide close regulation of the voltage, and such apparatus adds to the costs of construction and maintenance.
It is an object of this invention to provide methods and apparatus for significantly increasing the capacity and efficiency of electrostatic precipitators. Another object is to provide for operating a precipitator at high efficiency so that the size, weight and cost of the control apparatus are reduced as compared with existing apparatus. It is a further object to provide a precipitator which can be "tuned" to operate at optimum efficiency. An additional object is to obtain the effective use of low cost, small size precipitators at high collection efficiencies. Another object is to eliminate arcing or "breakdown", rather than merely to attempt to control it.
Various other objects and advantages of the invention will become apparent from the ensuing detailed description, and the novel features will be particularly pointed out in the claims.